Rolling-contact bearing and method of forming film of lubricant

ABSTRACT

A solid film of a fluorine-containing polyurethane polymer is formed on at least one of the constituent elements (inner and outer rings, rolling member and retainer) of a rolling-contact bearing. The molecules of the solid film of the fluorine-containing polyurethane polymer have urethane bonds, and form a three-dimensional reticular structure. Therefore, the film is unlikely to separate, peel off or wear. The occurrence of the dust can be restricted, and lubrication property can be improved.

TECHNICAL FIELD

The present invention relates to a roller bearing and a process offorming lubricating film therefor. The roller bearing will prove itsmerits when used in a vacuum, a clean atmosphere, or a corrosiveenvironment in which ordinary grease and oil cannot be used.

BACKGROUND ART

One of the above-mentioned special instances is the conveyor in thesemiconductor fabricating system. Grease is not adequate beause itevaporates to deteriorate the lubricating performance and to contaminatethe working environment. A common way of coping with this situation hasbeen to coat the raceway surface of the bearing ring, the surf ace ofthe rolling element, and/or the surface of the cage pocket with softmetal (such as gold, silver, lead, and copper) or solid lubricant (suchas carbon and molybdenum disulfide) in the form of film.

The coating film of solid lubricant has the disadvantage of graduallypeeling off while the rolling element is rotating or sliding. Althoughthe coating film evolves less dust than the grease, the resulting dustis detrimental to the clean working environment. In addition, the amountof dust increases under the high-load conditions.

The present inventors' experience in coating the bearing ring and cagewith a solid lubricant of fluoroplastic containing a binder revealedthat this solid lubricant evolves much less dust than the conventionalone. Nevertheless, the coating film of this solid lubricant is stillsubject to peeling and dropping (evolving dust) under a comparativelylarge axial load. The result of peeling and dropping is the decreasedlubricating function and the direct contact between metals, which leadsto adhesion, wear, and short life. In addition, with the coating filmpeeled off or dropped, the bearing is subject to corrosion in acorrosive environment.

It is an object of the present invention to provide a means to reducethe evolution of dust from a rolling bearing, thereby improving itslubricating performance and prolonging its life.

DISCLOSURE OF THE INVENTION

Constitution! The first aspect of the present invention resides in aroller bearing which is characterized in that at least one of itsconstituents is coated with a solid film formed from afluorine-containing polyurethane.

Incidentally, the solid film should preferably be one which has thethree-dimensional network structure. In addition, the solid film shouldpreferably be incorporated with a mobile fluoropolymer dispersedtherein. The mobile fluoropolymer should preferably be one which has nofunctional groups.

The second aspect of the present invention resides in a process offorming a lubricating film on a roller bearing.

The first embodiment of the process comprises the steps of dissolving ina solvent a fluoropolymer having isocyanate groups as functional groups,applying the solution to at least one of the constituents of a rollerbearing, thereby forming a liquid film thereon, and curing the liquidfilm, thereby forming a solid film of fluorine-containing polyurethaneof network structure.

The second embodiment of the process comprises the steps of dissolvingin a solvent a mixture of a fluoropolymer having isocyanate groups asfunctional groups and a fluoropolymer having at least one kind ofhydroxyl group, amino group, and carboxyl group, applying the solutionto at least one of the constituents of a roller bearing, thereby forminga liquid film thereon, and curing the liquid film, thereby forming asolid film of fluorine-containing polyurethane of network structure.

The third embodiment of the process comprises the steps of dissolving ina solvent a fluoropolymer having isocyanate groups as functional groupsalone or in combination with a fluoropolymer having at least one kind ofhydroxyl group, amino group, and carboxyl group, adding the solution toa solution of a fluoropolymer having no functional groups, applying thesolution to at least one of the constituents of a roller bearing,thereby forming a liquid film thereon, and curing the liquid film,thereby forming a solid film of fluorine-containing polyurethane ofnetwork structure in which the fluoropolymer having no functional groupsis dispersed in the mobile state.

The above-mentioned fluoropolymer may include perfluoropolyether havingno functional groups.

Functions! The fluorine-containing polyurethane mentioned above forms asolid film in which molecules are tightly connected to one another.Therefore, the solid film is hardly liable to peeling and wearing due tofriction between the rolling and sliding elements of the roller bearing.This leads to reduction in rolling and sliding resistance.

In the third embodiment of the process, the fluoropolymer remainingmobile in the solid film of the fluorine-containing polyurethane oozesout to contribute to the lubricating function.

Effects! The roller bearing of the present invention has an advantageover conventional ones because it has a solid coating film offluorine-containing polyurethane which is less liable to peeling,dropping, and wearing and hence is capable of reducing rolling andsliding resistance. The solid coating film contributes to lubricatingperformance, stable operation, and prolonged life. Therefore, it willfind use in the semiconductor manufacturing plant where precisionfabrication is required. Moreover, it will contribute to the improvedyields of semicondutors because it hardly contaminates the clean workingenvironment.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a vertical sectional view showing the upper half of the rollerbearing pertaining to one embodiment of the present invention.

FIG. 2 is a schematic diagram showing the molecular structure of thesolid film of fluorine-containing polyurethane formed on the rollerbearing shown in FIG. 1.

FIG. 3 is an absorption spectrum of the solid film of thefluorine-containing polyurethane which is not yet cured.

FIG. 4 is an absorption spectrum of the solid film of thefluorine-containing polyurethane which has been cured.

FIG. 5 is a schematic diagram showing a machine for testing samples inthe atmosphere.

FIG. 6 is a schematic diagram showing a machine for testing samples in avacuum.

FIG. 7 is a graph showing the results of test for torque life.

FIG. 8 is a graph showing the results of another test for torque life.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will be described in more detail with reference tothe examples (shown in FIGS. 1 to 8). FIG. 1 is a vertical sectionalview showing the upper half of the roller bearing. FIG. 2 is a schematicdiagram showing the molecular structure of the solid film offluorine-containing polyurethane formed on the roller bearing shown inFIG. 1. FIG. 3 is an absorption spectrum of the solid film of thefluorine-containing polyurethane which is not yet cured. FIG. 4 is anabsorption spectrum of the solid film of the fluorine-containingpolyurethane which has been cured. FIG. 5 is a schematic diagram showinga machine for testing samples in the atmosphere. FIG. 6 is a schematicdiagram showing a machine for testing samples in a vacuum. FIG. 7 is agraph showing the results of test for torque life. FIG. 8 is a graphshowing the results of another test for torque life.

The roller bearing (such as deep groove ball bearing) shown in FIG. 1 ismade up of an inner ring (1), an outer ring (2), spherical rollingelements (3), a wavy press-formed cage (4), and a solid film offluorine-containing polyurethane (5).

The inner and outer rings (1, 2), the rolling elements (3), and the cage(4) are made of corrosion-resistant material, such as JIS SUS440(martensitic stainless steel) and JIS SUS630 (precipitation hardeningstainless steel which has undergone hardening heat treatment). JISSUS304 (austenitic stainless steel) may be acceptable for light-loadapplications.

The corrosion-resistant metal for the inner and outer rings (1, 2) andthe rolling elements (3) may be replaced by ceramics, such as siliconnitride (Si₂ N₄), alumina (Al₂ O₃), silicon carbide (SiC), zirconia(ZrO₂), and aluminum nitride (AlN). The first one is used in combinationwith a sintering aid such as yttria (Y₂ O₃), alumina (Al₂ O₃), aluminumnitride (AlN), titanium oxide (TiO₂), and spinel (MgAl₂ O₄).

The cage (4) may be made of JIS SUS304 as well as brass and titanium. Itmay also be made of synthetic resins, such as polytetrafluoroethylene(PTFE), ethylene-tetrafluoroethylene (ETFE), polyether-ether ketone(PEEK), polyphenylenesulfide (PPS), polyethersulfone (PES), and nylon46. These synthetic resins may contain glass fiber for reinforcement.The cage (4) may take on a wavy form as well as a crown shape etc.

The inner and outer rings (1, 2), the rolling elements (3), and the cage(4) have their entire surface coated with the solid film (5) of thefluorine-containing polyurethane explained below.

The fluorine-containing polyurethane is composed of repeating unitsrepresented by the formula --C_(x) F_(2x) --O-- (where x is an integerof 1 to 4). It has an average molecular weight of millions and takes onthe three-dimensional network structure with molecules connected throughurethane linkage by the curing reaction. The three-dimensional networkstructure is a term used to express the chemical structure but it doesnot imply that the cross section of the film looks like a net. Itdenotes a structure in which molecules are uniformly and denselyconnected to one another like a network. Such a structure is formed bymodifying the chemical structure with a fluoropolymer having terminalisocyanate functional groups as shown in Formula (1) below.

     OCN--C.sub.6 H.sub.3 (CH.sub.3)--NHCO--CF.sub.2 .paren open-st.O--C.sub.2 F.sub.4 .paren close-st..sub.p .paren open-st.O--CF.sub.2 .paren close-st..sub.q OCF.sub.2 --CONH--(CH.sub.3)C.sub.6 H.sub.3 --NCO!.sub.n(1)

An example of the fluoropolymer having terminal isocyanate functionalgroups is a derivative of perfluoropolyether (PFPE).

The solid film (5) of the fluorine-containing polyurethane is formed inthe following way.

(a) Dipping: A solution (1 mass %) of fluorine-containing polyurethaneis prepared by dissolving a derivative of perfluoropolyether in afluorine-based solvent (SV90D). In this solution are dipped the innerand outer rings (1, 2), the rolling elements (3), and the cage (4)separately or in the form of assembly. (In the second case, the bearingis rotated several times in the solution.)

(b) Drying: Dipping is followed by heating at 40-50° C. for about 1minute for solvent removal. The coating film remains mobile in thisstage.

(c) Curing: The coating film is cured by heating at 100-200° C. for 20hours so that the liquid film chemically changes into the solid film (5)of fluorine-containing polyurethane.

The curing reaction causes the molecules of fluorine polymer to form thenetwork structure through urethane linkage with the elimination ofterminal isocyanate groups (NCO) according to the following fourchemical reactions (Formulas 2 to 5). The urethane linkage extends themolecules as shown in FIG. 2(a) and then forms the three-dimensionalcrosslinks as shown in FIG. 2(b).

Incidentally, the molecule represented by formula (1) is represented byY-X˜X-Y in FIG. 2, X and Y denoting NHCO and NCO, respectively, inFormula (1). ##STR1##

The above-mentioned procedure makes it possible to form the solid filmof fluorine-containing polyurethane in an adequate thickness on theconstituents of the roller bearing. According to the use of the bearing,the thickness of the solid film may be adjusted in the range of 0.1 to 3μm by repeating the steps (a) and (b) above as many times as necessary.

To examine the effect of curing, the following experiments wereconducted. In the first experiment, the solution obtained in step (a)was simply dried and the resulting film (still mobile) was analyzed byFourier transform infrared spectroscopy (FT-IR, liquid film method). Thesample gave a spectrum shown in FIG. 3. There are peaks due to NH (3300cm⁻¹), N═C═O (2279 cm⁻¹), N(H)C═O (1712 cm⁻¹, 1546 cm⁻¹), and benzene(1600 cm⁻¹) in addition to peaks due to fluorocarbon. These peakssuggest the presence of benzene rings, urethane linkages, and isocyanategroups. The spectrum is the same regardless of the thickness of thefilm. In the second experiment, the above-mentioned solution was appliedto a stainless steel sheet and the coating film was cured. The curedfilm was analyzed by Fourier transform infrared spectroscopy (FT-IR,reflection method). The sample gave a spectrum shown in FIG. 4. Thereare peaks due to benzene rings and urethane linkages, but there are nopeaks due to isocyanate. These results support the chemical changes offunctional groups by curing as shown in Formulas 2 to 5 above.

The solid film (5) of the fluorine-containing polyurethane has thethree-dimensional network structure, densely covers the substrate, andexhibits the self-lubricating properties. Therefore, it prevents thebearing constituents from coming into direct contact with each other andit does not wear, peel off, and evolve dust while the roller bearing isrunning.

According to another embodiment of the present invention, the solid film(5) of the fluorine-containing polyurethane is formed such that mobilemolecules of a fluoropolymer (such as fluoropolyether) are dispersed inthe three-dimensional network structure of urethane linkage. This solidfilm may be formed from a solution of a fluorine-containing polymerhaving terminal isocyanate functional groups and a fluoropolymer havingno functional groups. Since these two polymers do not combine with eachother in the curing stage (c), the latter remains mobile in the solidfilm (5) and oozes out from the solid film (5) to contribute to thelubricating action. The fluorine-containing polymer may containfunctional groups as shown in Formulas 6 to 8 below.

    HO--CF.sub.2 --CF.sub.2 .paren open-st.O--C.sub.2 F.sub.4 .paren close-st..sub.p .paren open-st.O--CF.sub.2 .paren close-st..sub.q OCF.sub.2 --CH.sub.2 --OH                                 (6)

    H.sub.2 N--CH.sub.2 --CH.sub.2 --NHCO--CF.sub.2 .paren open-st.O--C.sub.2 F.sub.4 .paren close-st..sub.p .paren open-st.O--CF.sub.2 .paren close-st..sub.q --OCF.sub.2 --CONH--CH.sub.2 --CH.sub.2 --NH.sub.2 (7)

    HOOC--CF.sub.2 .paren open-st.O--C.sub.2 F.sub.4 .paren close-st..sub.p .paren open-st.O--CF.sub.2 .paren close-st..sub.q OCF.sub.2 --COOH(8)

Examples 1 to 4 and Comparative Example 1 that follow demonstrate theeffect of the solid film of fluorine-containing polyurethane on dustevolution and torque life.

In Examples 1 to 4, the solid film (1 μm thick) of fluorine-containingpolyurethane is formed on the entire surface of the inner and outerrings, rolling elements, and cage.

In Example 1, the fluorine-containing polyurethane which has isocyanategroups as terminal functional groups.

In Example 2, the fluorine-containing polyurethane is a mixture of afluorine-containing polyurethane which has isocyanate groups as terminalfunctional groups, and a fluorine-containing polyurethane which hashydroxyl groups (--OH) as terminal functional groups.

In Example 3, the fluorine-containing polyurethane is a mixture of afluorine-containing polyurethane which has isocyanate groups as terminalfunctional groups, and a fluorine-containing polyurethane which has nofunctional groups.

In Example 4, the fluorine-containing polyurethane is a mixture of afluorine-containing polyurethane which has isocyanate groups as terminalfunctional groups, a fluorine-containing polyurethane which has hydroxylgroups (--OH) as terminal functional groups, and a fluorine-containingpolyurethane which has no functional groups.

In Comparative Example 1, a solid coating film (1 μm thick) of polyimide(containing polytetrafluoroethylene (PTFE) dispersed therein) is formedon the entire surface of the inner and outer rings and cage. Thiscoating film is composed of polyimide (which is a comparatively hard anddense thermoplastic resin) as the matrix and polytetrafluoroethylene asdispersed particles. Bond between the two components is weak.

The samples are tested in the atmosphere using the apparatus shown inFIG. 5 and in a vacuum using the apparatus shown in FIG. 6. In FIGS. 5and 6, there are shown bearing samples (50, 50), a rotating shaft (51),a casing (52), a magnetic fluid seal (53), a dust particle counter (54),a recorder (55), a bearing housing (56), and a coil spring to exert anaxial load (57).

In both Example and Comparative Example, the bearing sample is SE608(φ8×φ22×7), with inner and outer rings made of JIS SUS440C and cage(wavy type) made of JIS SUS304. The raceway of the inner and outer ringshas a surface roughness of 0.1 Z, and the rolling element has a surfaceroughness of 0.05 a. Test conditions are as follows:

Rotary speed: 200 rpm

Load: axial load (20 N, 25 N, 50 N, 100 N)

Atmosphere: Clean bench (class 10) Vacuum (2.6×10⁻⁴ Pa or less)

Temperature: room temperature, high temperature (200° C.)

Particle counting: Particles (dust) larger than 0.1 μm

The bearing samples were tested for time required for the particle(dust) counting reaches a prescribed number and also tested for timerequired for the torque to reach a prescribed level.

(1) Dust counting test: In this test, the bearing is run in theatmosphere at room temperature under a load of 100 N, and time requiredfor the counting of particles (dust) to exceed 1000/0.1 cf. Measurementsare repeated 10 times at intervals of 10 minutes. The coating film ismade from the solution specified below.

In Example 1: a fluorine-containing polyurethane which has isocyanategroups as terminal functional groups, 1 mass %.

In Example 2: a fluorine-containing polyurethane which has isocyanategroups as terminal functional groups, 1 mass %; a fluorine-containingpolyurethane which has hydroxyl groups (--OH) as terminal functionalgroups, 0.25 mass %.

In Example 3: a fluorine-containing polyurethane which has isocyanategroups as terminal functional groups, 1 mass %; a fluorine-containingpolyurethane which has no functional groups, 0.25 mass %.

In Example 4: a fluorine-containing polyurethane which has isocyanategroups as terminal functional groups, 1 mass %; a fluorine-containingpolyurethane which has hydroxyl groups (--OH) as terminal functionalgroups and a fluorine-containing polyurethane which has no functionalgroups, 0.25 mass % in total.

The results are as follows:

Example 1 . . . 6 hours

Example 2 . . . 10 hours

Example 3 . . . 31 hours

Example 4 . . . 30 hours

Comparative Example 1 . . . 4 hours

It is noted that Example 1 in which a fluorine-containing polyurethanewhich has isocyanate groups as terminal functional groups, is used aloneas the fluorine-containing polyurethane is superior to ComparativeExample 1 but is inferior to Examples 2 to 3 in which afluorine-containing polyurethane which has isocyanate groups as terminalfunctional groups, is used in combination with a mobile fluoropolymer.

The good results in Examples 1 to 4 are due to the fact that the solidfilm (5) of fluorine-containing polyurethane is a dense, uniform film ofthree-dimensional structure and hence is less liable to peeling andwearing while the constituents of the roller bearing are rolling andsliding.

The results of the dust counting test under different axial loads are asfollows.

    ______________________________________    Axial load       50N    20N    ______________________________________    Example 3        120 h  335 h    Example 4        115 h  320 h    ______________________________________

This holds true also for Examples 1 and 2. In Comparative Example 1, theresult of the dust counting test is improved from 4 hours (under a loadof 100 N) to 10 hours (under a load of 50 N). This result is still muchpoorer than that in Example 3. Therefore, it is recommended that any ofthe solid films specified in Examples 1 to 4 should be selectedaccording to the operating conditions.

It is concluded from the foregoing that the solid film (5) shouldpreferably be formed from a fluorine-containing polymer having terminalisocyanate functional groups in combination with a mobile fluoropolymer.The mobile fluoropolymer should preferably be one which has nofunctional groups.

To examine the effect of concentration, the dust counting test (under aload of 100 N) was carried out in such a way that the concentration of afluorine-containing polyurethane which has isocyanate groups as terminalfunctional groups, (as the base material) was 1 mass % and theconcentration of a fluorine-containing polyurethane which has nofunctional groups (as the secondary material) was 0.25 mass % or 0.5mass %. The results were 31 hours for 0.25 mass % and 22 hours for 0.5mass %, with the former concentration being desirable. In practice, theconcentration of a fluorine-containing polyurethane which has nofunctional groups may range from 0.1 to 0.75 mass %, and theconcentration of a fluorine-containing polyurethane which has isocyanategroups as terminal functional groups, may range from 1 to 10 mass %. Itis desirable to use them in a certain ratio regardless of theconcentration. Time in the dust counting test tends to decrease as theconcentration of a fluorine-containing polyurethane which has isocyanategroups as terminal functional groups, (as the base material) increases.

(2) Torque test: This test was performed on the samples in Example 3 andComparative Example 1 in a vacuum at room temperature under an axialload of 25 N or 50 N. The results are shown in FIG. 7. (The test wassuspended after 600 hours.) It is apparent that the coating reducestorque by nearly half as summarized below.

    ______________________________________    Axial load       25N         50N    ______________________________________    Example 3        3-4 N · m                                 5-6 N · m    Comparative Example 1                      8-10 N · m                                 12-15 N · m    ______________________________________

Almost the same results as above were obtained with samples in Examples1 and 2.

The torque test was also carried out in a vacuum at 200° C. under anaxial load of 50 N. The result is shown in FIG. 8. It is noted that thesample in Comparative Example 1 showed a sharp increase in torque after10 hours, whereas the sample in Example 3 remained unchanged in torqueover 90 hours. This result suggests that the coating film prolongs thelife of the bearing regardless of temperature. The reason for this isthat the two components for the coating film do not combine with eachother but remain mobile and exhibit the lubricating action.

The present invention is not limited to the above-mentioned examples;but various changes and modifications may be made as follows.

(1) The deep groove ball bearing used in Examples may be replaced byroller bearings of any other type which may not have the cage.

(2) The curing may be carried out by irradiation (with ultraviolet rays,infrared rays, gamma rays, or electron rays) in place of heating.

(3) The step of drying may be omitted.

(4) It is not always necessary to coat all the constituents of thebearing; but it is possible to limit the coating to the inner and outerrings (1,2) or the rolling elements (3) alone.

(5) It is not always necessary to coat the entire surface of theconstituents of the bearing; but it is possible to limit the coating tothe raceway surface of the inner and outer rings (1,2) or that part ofthe cage (4) which comes into contact with the rolling elements (3).Such partial coating may be accomplished by masking prior to dipping.However, the entire coating is desirable for corrosion prevention andobviates the necessity of anticorrosive treatment.

We claim:
 1. A roller bearing which is characterized in that at leastone of its constituents is coated with a solid film formed from afluorine-containing polyurethane polymer.
 2. A roller bearing as definedin claim 1, wherein the solid film has a three-dimensional networkstructure.
 3. A roller bearing as defined in claim 1 or 2, wherein thesolid film contains a mobile fluoropolymer dispersed therein.
 4. Aroller bearing as defined in claim 3, wherein the mobile fluoropolymerhas no functional groups.
 5. A process of forming a lubricating film ona roller bearing, said process comprising the steps of dissolving in asolvent a fluoropolymer having isocyanate groups as functional groups,applying the solution to at least one of the constituents of a rollerbearing, thereby forming a liquid film thereon, and curing the liquidfilm, thereby forming a solid film of fluorine-containing polyurethanepolymer of network structure.
 6. A process of forming a lubricating filmon a roller bearing, said process comprising the steps of dissolving ina solvent a mixture of a fluoropolymer having isocyanate groups asfunctional groups and a fluoropolymer having at least one of a hydroxylgroup, an amino group, and a carboxyl group, applying the solution to atleast one constituent of a roller bearing, thereby forming a liquid filmthereon, and curing the liquid film, thereby forming a solid film offluorine-containing polyurethane polymer of network structure.
 7. Aprocess of forming a lubricating film on a roller bearing, said processcomprising the steps of dissolving in a solvent a fluoropolymer havingisocyanate groups as functional groups alone or in combination with afluoropolymer having at least one of a hydroxyl group, an amino group,and a carboxyl group, adding the solution to a solution of afluoropolymer having no functional groups, applying the solution to atleast one constituent of a roller bearing, thereby forming a liquid filmthereon, and curing the liquid film, thereby forming a solid film offluorine-containing polyurethane polymer of network structure in whichthe fluoropolymer having no functional groups is dispersed in a mobilestate.
 8. The roller bearing according to claim 1, wherein said solidfilm comprises a fluoropolymer having an isocyanate group.
 9. A rollerbearing according to claim 8, wherein solid film comprises a mixture ofa fluoropolymer having an isocyanate group and a fluoropolymer having ahydroxyl group, an amino group, or a carboxyl group as a terminalfunctional group.
 10. The roller bearing according to claim 9, whereinsaid solid film further comprises a fluoropolymer having no functionalgroup.
 11. The roller bearing according to claim 8, wherein said solidfilm comprises 1-10 mass % of a fluoropolymer having an isocyanategroup.
 12. The roller bearing according to claim 10, wherein said solidfilm comprises 1-10 mass % of a fluoropolymer having an isocyanategroup, and 0.1 to 0.75 mass % of a fluoropolymer having no functionalgroup.
 13. The process according to claim 5, wherein said solid filmcomprises 1-10 mass % of a fluoropolymer having an isocyanate group. 14.The process according to claim 7, wherein said solid film comprises 1-10mass % of a fluoropolymer having an isocyanate group, and 0.1 to 0.75mass % fluoropolymer having no functional group.
 15. The roller bearingas defined in claim 1, wherein said solid film is formed by curing aliquid film comprising the fluorine-containing polyurethane polymer byheating at 100-200° C. for 20 hours.
 16. The process according to claim5, wherein said liquid film is cured by heating at 100-200° C. for 20hours.